CATIONIC POLYMERIZATION OF ETHYLENE OXIDE

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1 CATIONIC POLYMERIZATION OF ETHYLENE OXIDE IV. THE PROPAGATION REACTIONS1 ABSTRACT Studies of the boron-fluoride-catalyzed reaction of ethylene oxide with simple alcohols and low molecular weight polyglycols indicate three ways in which chain growth could occur, but only one of these is considered to be important after the initial stages of polymerization. The rate of disappearance of monomer reaches a maximum at a molecular weight of about 400. A mechanism is proposed to account for both polymerization and depolymerization and it is shown how equilibrium between these two reactions could result. The polymerization of ethylene oxide by boron fluoride is a complex process in which the propagation reaction must compete with an appareiltly independent depolymerization reaction. The depolymerization process, as discussed in the previous paper (I), breaks down the polyglycol to dioxane seemingly through a chain reaction propagated by oxonium ions acting upon the ether linkages in the polymer molecule. The polymerization on the other hand appears to occur through a stepwise addition of monomer to the terminal hydroxyls of the polymer chain. A study was therefore undertaken of the reaction of ethylene oxide with alcohols in the presence of boron fluoride in an effort to obtain a mechanism for the propagation and to observe the transition fro111 propagation to depolymerization. This paper presents the results of these studies.. EXPERIMENTAL Ethylene chloride, ethylene oxide, and boron fluoride were prepared for use as previously described (2). Volatile alcohols were dried by distillation froill their magnesium alcoholates in the usual way and ethers were dried over sodium metal or calcium hydride, \ZTherever the reactants were sufficiently volatile they were stored on the vacuum system and transferred by distillation. The polyglycols were either stirred under high vacuum for several hours or dissolved in dry ethylene chloride and pumped free from solvent before use. Molecular weights were determined by viscosity ineasureme~lts as before, but in a few cases were confirined by freezing point depressions in benzene. Where hydroxyl content of the polynler was important, end group analysis by acetylation with acetyl chloride in pyridine, according to the nlethod of Smith and Bryant (3), was used. All three methods gave satisfactory agreement. Reactions were carried out in ethylene chloride solution at 20 C. Rates were followed either by the decrease in vapor pressure of ethylene oxide as described in the earlier work, or by a dilatometric method. The dilatometers were so constructed that they could be dried and filled under vacuunl and then transferred to a constant temperature bath while exposing only the capillary tip to the atmosphere. Absolute contractions were not determined but the contraction per unit of oxide appeared to be constant over the range of the experiments. The two methods gave good agreement wherever they could be compared. 'Manuscript received May 13, Contribution from the Division of Applied Chemistry, National Research Council, Ottawa, Canada. Issued as N.R.C. No Can. J. Chem. Vol. 38 (1960) 1967

2 CANADIAN JOURNAL OF CHEMISTRY. VOL GO RESULTS AND DISCUSSION Before discussing the reaction of ethylene oxide with alcohols, it is necessary to draw some conclusions as to the probable distribution of boron fluoride in a system containing both hydroxyl and ether groups. We know (4) that boron fluoride with water forms both non no hydrates and dihydrates and that the second molecule of water is held, at least in the crystal, as the hydronium ion, The dihydrate is much the more stable of the two complexes but in the corresponding alcoholates this difference in stability appears to be much smaller. Ethers yield 1:l compounds with boron fluoride but in the presence of alcohols they yield ternary complexes, ailalogous to the dihydrates, in which the ether is held by hydrogen bonding, ROH: BF3 + R;O S RO:BF3... HOR; E ROH + R;O: BF3. Obviously then, in a solution of hydroxyl and ether groups such as one has with the polyglycols, the boron fluoride will be present as an equilibrium mixture of several complexes and, since the reaction rates will depend on the concentrations and reactivities of the various species present, a rigorous interpretation of the kinetics will require all accurate knowledge of all the equilibriunl constants. Information of this kind is not available nor is it likely to be for some time to come so a few simple experiments were conducted in an effort to obtain some idea of the relative importance of the various complexes. The relative basicities of a few ethers towards boron fluoride were examined b>equilibrating the ethers in ethylene dichloride solution against dimethyl ether and boron fluoride. The concentration of free dimethyl ether in solution was obtained from its vapor pressure and the position of the equilibrium was then calculated from the known initial concentrations of all reactants. Constants obtained in this way held very well over about a fivefold range of conce'ntrations and, where comparison could be made, were in good agreement with values calculated from the data of Brown and Adams (5) for the thermal dissociation of boron fluoride etherates. A few values of the relative basicities are listed in Table I. TABLE I Dimethyl ether Di-n-butyl ether Dioxane (molar basis) Tetrah ydrofurane Ethylene oxide unfortunately could not be examined because of its high reactivity, but the dimethyl ether of diethylene glycol, CH30CH2CH20CH2CH20CH3, is of some interest because of its siinilarity to the polyglycols. In our experiments this molecule was found to add only two molecules of boron fluoride and of these one seemed to be held quite loosely. At low boron fluoride concentrations, however, the triether was quite basic with a relative basicity, on a molar basis, of about 0.4. In a similar manner a little information was obtained about the ternary complexes. If one adds dimethyl ether to a solution of equimolar amounts of boron fluoride and n-butanol, then an amount of ether equal to

3 MERRALL ET AL.: POLYMERIZATION OF ETHYLENE OXIDE. 1V 1969 the boron fluoride is removed from the solution. Since addition of further alcohol regenerates some ether, an equilibrium of the type ROBF,... H... OR: + ROH ROBF,... H... HOR + R40 seems indicated so, 011 the assumption that any binary complexes present are in low concentration compared with the ternaries, an equilibrium constant can be obtained. This constant was found to hold surprisingly well over the short range of concentrations which could be examined and indicated that the dimethyl ether was about 1.5 times as basic as n-butanol towards the acid ROH:BF3. From subsequent experiments it seems likely that at high ratios of ether to alcohol serious interference from the binary ether - boron fluoride complex may occur, but as much of our work has been carried out at lowvalues of this ratio, we feel justified in assuming that the catalyst will be present largely as the ternary complex in most of our reaction mixtures. Furthermore, as alcohols and ethers seem to have rather similar basicities, we must expect that a mixture of ternary complexes will be present. Ethylene oxide reacts with simple alcohols to give the P-alkoxyethanols which can in turn react with further monomer. Since the latter reaction is several times more rapid than the initial one, rates must be obtained either from initial slopes or from experiments at high alcohol/oxide ratios. The results obtained with n-butanol are shown in Table I1 TABLE I1 Reaction of n-butanol with ethylene oxide Initial rate, moles liter-' min-' Butanol Oxide BF3 Observed Calculated Vapor pressure method 1.10, , Dilatometric method

4 1970 CANADIAN JOURNAL OF CHEMISTRY. VOL but essentially similar results were obtained with ethanol. The rate of the reaction decreases with increasing alcohol concentration. From our previous discussion it seems most probable that this decrease is due to an increase in the concentration of boron fluoride dialcoholate, so we may suppose that the rate is governed by the following reactions, KI -2 ROH + ROH. BFj. oxide -- ROH - BF,. ROH + oxide \ / Leact ion 121 reaction [I] k1 \ J k 2 product where reaction [I] is the unimolecular rearrangement of a ternary complex of all three reactants while reaction [2] is a bimolecular reaction between ethylene oxide and the boron fluoride dialcoholate. The rate expression for this system, assuming only ternary complexes, is found to be --- d (oxide) kiki+ k2 (ROH) dt - (BFs) (oxide) Kl(oae)+(ROH) which, when assigned the following values for the constants, K1 = 0.1, k1 = 9 min-i, and k2 = 0.4 liter mole-l min-i, gives the calculated rates shown in Table 11. The agreement with the experimental values is good but since there are three constants to adjust such agreement does not mean a great deal. The real test of the mechanism would perhaps be an independent measurement of the equilibrium constant K1, i.e. of the basicity of ethylene oxide and this, as noted above, we were unable to obtain. The above kinetic value of K1 suggests that the oxide is about l/loth as basic as n-butanol and hence about 15 times less basic than dimethyl ether, so is in agreement with qualitative experiments (6) which indicate that epoxides are among the least basic of the simple ethers. The reaction of ethylene oxide with a simple alcohol is the first step in an addition polymerizatioil in which the second and third steps can be represented by the reactions of the monoethyl ethers of glycol (Cellosolve) and diethyleile glycol (Carbitol), respectively. The results of some dilatometric experiments with these alcohols are shown in Fig. 1; vapor pressure measurements were in reasonable agreement with the dilatometric measurements at the higher alcohol concentrations but diverged at the lower ones. The rate measurements show a minimum with both Cellosolve and Carbitol at low alcohol concentration. We have examined this region carefully and feel that the minimum is probably real but admittedly it could be due to a breakdown of the dilatometric method in this region or to experimental errors which tend to accumulate at low alcohol concentrations. If it is real, it would appear that the increasing rate at very low hydroxyl concentrations corresponds to an extension of the curve for n-butanol to low alcohol concentrations, i.e. to a region where reaction [I] is dominant. This view receives some support from the fact that no minimum is evident in runs with a polyglycol of molecular weight 600 (Fig. 1) where the contributioil from reaction [I] would be reduced markedly by the competition of the polymer ether groups for the boron fluoride. While the presence of the minimum is disturbing, its presence should not be allowed to obscure the most important conclusion from these results, namely, that a clear change in mechanism, not

5 MERRALL ET AL.: POLYMERIZATION OF ETHYLENE OXIDE. IV L 1 I I I I I ae Lo 12 HYDROXYL CONCENTRATION merely a change in rate, has occurred in passing from the simple alcohols to alcohols. This change must be due to the presence of the ether groups and suggests that an oxonium ion type of reaction is important even at this early stage of the polymerization. Meerwein 'and his co-workers have shown (7) that epoxides react, both rapidly and quantitatively, with anhydrous ethers and boron fluoride to form the iililer oxonium salts, which can then react with further etherate to form the tertiary salts, The inner salts differ from the tertiaries in being very unstable and insoluble so most probably account.,for the precipitatioil and discoloration we observe in the absence of

6 1972 CANADIAN JOURNAL OF CHEMISTRY. VOL. 38, 1960 hydroxyl groups. It may be that the hydroxyl groups decompose the inner salts as formed, but an alternative explanation is possible and would account for other aspects of the polymerization process. Klages and Meuresch (8) showed that ether complexes of very strong acids will react with diazomethanes to form tertiary oxoilium salts, both rapidly and quantitatively, The acid-ether complexes here are closely similar to the alcohol-ether - boron fluoride complexes ellcountered in the present work so if ethylene oxide could replace the diazomethane, tertiary oxonium ions would be formed directly according to the reaction RO: BFa H k OR; + CHI-CH2 --r R;~cH~cH~oH BFIOR \o/. [41 A reaction of this forin seems best able to account for the rapid reaction of ethylene oxide with the lower ether alcohols and may be considered as an exact analogue of reaction [2], differing from it only in the stability of the product; in reaction [2] the catalyst is regenerated immediately whereas in reaction [A] the oxonium ion is sufficiently stable that it must be decomposed by reaction with a hydroxyl group, i.e. - k s R!~CH,CH~OH + ROH R H+ ~O + R A CH2CH20H. [51 The net effect of this formatioil and destruction of the oxonium ion is to lengthen a chain by one oxide unit so we now have three processes which can lead to chain growth, reactions [I], [2], and [4, 51, all of which must be considered as important in the reaction of Cellosolve and Carbitol. The system is obviously too coinplex for complete kinetic analysis but under conditions of high alcohol/oxide ratios, as in some of the dilatometric experiments, reaction [I] should not be important and the boron fluoride will be distributed between two ternary complexes, ROH. BF~*alcohol/ROH* BF3.ether = Ko, according to whether hydrogen bonding takes place at, an alcohol or ether group. The value of K2 will be governed by the ratio of ether to alcohol groups in the moleculeand not by the coilcentration of the ether-alcohol; it will decrease as the chain grows longer. If steady-state conditions were to apply to the oxonium ion, the rate expression for the system would be - d (oxide) = k'k2+k4k5 /RP., /nv:an\ or if reaction [5] were very rapid compared with [4] - d (oxide) - dt k'k2+ k4 (BF3) (oxide). K2+ 1

7 MERRALL ET AL.: POLYMERIZATION OF ETHYLENE OXIDE. IV 19'73 These very similar expressions are in agreement with the observed first-order dependence 011 boron fluoride and ethylene oxide but require that the rate be independent of hydroxyl concentration. It is evident in Fig. 1 that at the higher alcohol concentratioils where reaction [I] can be disregarded, the rates do in fact tend towards such independence without actually achieving it. This failure of the rate expressio~ls is probably due to the high rate of reaction [i], which invalidates the assumptions used in deriving them. Extension of these results to low molecular weight polymers is not simple. Rate expressions of the type just considered suggest that as the molecular weight increases and K2 beco~lles very small, the reaction should reduce to a simple second-order process. In fact, however, at high molecular weights ( ) the rate of oxide disappearance becomes very slow and independent of inonoiner concentration. For reasons detailed in the previous paper (I), we believe that this slow zero-order rate is due to the formation of stable oxonium ions which do not react directly with ethylene oxide with the result that reaction [5] becomes rate controlling. (In solutions of high ~nolecular weight polyn~er the hydroxyl concentration is, of necessity, low.) Between the two extremes of the very low and the high molecular weight polyglycols the rate of reaction of oxide with the glycols, at constant hpdroxyl concentration, passes through a illaxirnunl at a inolecular weight of about 400. The order in oxide at moleculer weights below the maximum is always one or nearly so, but at higher molecular weights it drops off steadily. If, as we believe, the order is determined by the relative rates of reactions [4] and [5], then it is legitinlate to couipare initial rates of oxide disappearance and this is done in Fig. 2 for initial concentrations of boron fluoride and ethylene oxide of and 0.55 M, respectively. I I I I I I I I I MOLECULAR WEIGHT OF GLYCOL FIG. 2. It was stated earlier that the increase in rate in passing from the siinple alcohols to the ether alcohols is most probably due to a contribution from a reaction proceeding via oxonium ions. On the other hand, the decrease in both rate and order at higher molecular weights is almost certainly due to the presence of stable oxonium ions, so the problem is to resolve this apparent conflict in a way which will account for the rate maximum at iiiolecular weight 400. Dilatometric experiments with low molecular weight polymer and very small amounts of monomer suggest strongly that reaction [4], oxonium ion formation,

1874 CANADIAN JOURNAL OF CHEMISTRY. VOL. 38. 1960 is very fast even at molecular weights well above 400 (Fig. 1), so the over-all rate must be governed largely by reaction [5].

8 1874 CANADIAN JOURNAL OF CHEMISTRY. VOL is very fast even at molecular weights well above 400 (Fig. 1), so the over-all rate must be governed largely by reaction [5]. It follows that reaction [5] must be especially rapid in the molecular weight range and since there seems no reason why reaction between an oxonium ion and any terminal hydroxyl group should be molecular weight dependent, this conclusion suggests that internal reaction, i.e. reaction of the oxonium ion with the hydroxyls of its own chain, is important at low molecular weights. Once reached, this conclusion seems quite reasonable for we know that an oxonium ion readily attacks the oxygen atoms of its own chain (dioxane formation), so with low molecular weight polymer there is a high probability that the oxygen attacked will be that of a hydroxyl group. With triethylene glycd, for example (HOCH2CH,OCH2CH2OCH,CH2OH, mol. wt. 150), no other reaction is possible, but with higher polymers one or more dioxane units may have to be eliminated before the attack at hydroxyl is possible. The reaction could take place in two ways, one leading to polymerization and the other to depolymerization, ROCH2CHZ /CHCH2\ + ROCH2CH2, /CH2CHr\ OHf O HOCHCH~/ \CHICHA depolymerization HOCHEH/ H" 'CH~CH? I I H+ - - ROCH2CH20CH2CH20CH2CH20CHzCH20H polymerization but from the point of view of rate it does not matter which occurs since both lead to destruction of the oxonium ion. Our evidence suggests, however, that polymerization is favored for we could detect no dioxane in the product of the reaction between the monoethylether of triethylene glycol and an equimolar amount of triethyloxonium fluoroborate. According to the reaction scheme here proposed, the increasing rate at low molecular weights is due primarily to a transition from reaction [2] to reaction [4]. With continued chain growth, however, reaction [5], which is composed of two parts corresponding to internal and external termination, becomes steadily slower as internal termination becomes less and less probable. Since the probability of internal termination should be inversely related to the molecular weight, it is not surprising that the maximum rate occurs at a definite molecular weight at all hydroxyl concentrations. More surprising perhaps is the relatively small contribution of the external to the total termination. With a reaction mechanism for guidance, the conditions governing molecular weight equilibrium can be considered. If equilibrium were to occur at molecular weights above about 1500, the situation would be relatively simple because it could be assumed that all termination occurred by tlie external route and the order in oxide could be taken as zero, with all the catalyst in the form of oxonium ion. The rates of dioxaile formation and nlorlorner disappearance would be given approximately b~r and would be related at equilibrium as d(dioxane)/dt = k6(oxonium ion), - d(oxide)/dt = k.r(oxoniun1 ion) (ROH), It follows that equilibrium would occur at a hydroxyl concentration equal to 2k6/k7 but it would be an equilibrium in the sense that if a polymer were added to the reaction so as to give a hydroxyl concentration of 2k6/k7, then that polymer would neither grow nor depolymerize. At higher hydroxyl concentrations growth would occur and at lower concentrations the polymer would disappear.

9 MERRALL ET AL.: POLYMERIZATION OF ETHYLENE OXIDE. IV 1975 In practice equilibrium occurs at n~olecular weights where the order in oxide is less than about 0.5 but certainly not zero, and where internal termination is still a major part of the total termination. If the oxide concentration were important in determining the equilibriunl, then the molecular weight should rise to a maximum in the course of a polymerization and fall off again as the monomer concentration declines. We have found some evidence for a maximum of this kind but it is scarcely greater than the error in the weight measurements, so the concentration of oxide will be ignored in the present discussion. The contribution of internal termination to the equilibriunl, on the other hand, cannot be ignored and unfortunately cannot be expressed with our present inforinatioti. It obviously will become smaller as the molecular weight increases and may perhaps be roughly inversely proportional to the molecular weight, in which case the total termination rate would be given by - d(oxonium ion)/dt = [k7(roh) + k8/(mol. wt.)](oxoniuln ion) which in turn, assunling that reaction [5] is rate controlling, would be the rate of oxide disappearance. Accordingly at equilibrium 2k6(oxoniu.m ion) = [k7(roh) +k8/(mol. wt.)](oxoniunl ion) This treatilleilt is obviously exceedingly crude but is interesting in that it suggests that the equilibrium molecular weight should be somewhat dependent on the hydroxyl concentration for this in fact seems to be the case (Table 111). TABLE I Hydroxyl concentration Approx. final molecular wt. X reaction as complex as this one will require a great deal of work before it yields ally exact solutioils and we do not pretend that the reaction schemes presented in this and the preceding paper are any more than preliminary ones. They do, however, seen1 to accouiit in a qualitative and fairly logical way for almost all of our observations 011 the boron fluoride catalyzed polymerization of ethylene oxide. Perhaps the most interesting problem which remains unexplained is the difference in behavior of the various Friedel-Crafts catalysts in this polymerization, for the reaction schemes proposed do not seein to account satisfactorily for the fact that stannic chloride, and perhaps antimony pentachloride, can produce much higher molecular weight polymer than boron fluoride. REFERENCES 1. G. A. LATR~~MOUILLE, G. T. MERRALL, and A. M. EASTHAM. J. Am. Chem. Soc. 82, 120 (1960). 2. D. J. ~VORSFOLD and A. M. EASTHAM. J. Am. Chem. Soc. 79,900 (1957). 3. D. M. SMITH and W. M. D. BRYANT. J. Am. Chem. Soc. 57, 61 (1935). 4. N. N. GREENWOOD and R. L. MARTIN. Quart. Rev. 8, 1 (1954). 5. H. C. BROWN and R. M. ADAMS. J. Am. Chem. Soc. 64, 2557 (1942). 6. S. SEARLES and M. TAMRES. J. Am. Chem. Soc. 73, 3704 (1951). 7. H. MEERWEIN, E. BATTENBURG, H. GOLD, E. PFEIL, and G. WILLFANG. J. prakt. Chem. 154,83 (1939). 8. F. KLAGES and H. MECRESCH. Ber. 85, 863 (1952); 86, 1322 (1953).

GO-CATALYSIS IN FRIEDEL-CRAFTS REACTIONS

GO-CATALYSIS IN FRIEDEL-CRAFTS REACTIONS VI. POLYMERIZATION OF 2-BUTENE BY BORON FLUORIDE - METHANOL'. H. R. ALL COCK^ AND A. M. EASTHAM Division of Applied Chemistry, National Research Council, Ottawa,